Talc is a mineral which has many industrial applications: as a filler in paper, as a paper coating pigment, as an additive for the control of pitch and resin in paper ("pitch control"), as a reinforcing filler for thermoplastics, a filler for paint, a thixotropic additive, an anti-clumping additive, a cosmetic base, a raw material in the manufacture of ceramics, etc. However talc is a hydrophobic and inert mineral, properties which result in some disadvantages in certain existing applications, which can restrict its field of use. For example, for applications in the paper industry, the hydrophobic nature of talc complicates mixing procedures in aqueous media and weakens its bond to cellulose, which results in the occurrence of powdering (the release of talc from the surface of the paper). In applications as a filler in plastics materials the inert nature of the talc prevents it from bonding tightly to the plastics matrix, which limits certain mechanical properties of the filled composite.
At the present time the best known process for masking the hydrophobic and inert properties of talc is to encapsulate each particle in an envelope having the desired properties (P. Godard, J-P. Mercier: Double Liaison-Chimie des Peintures, (387) 3/19 (1988). R. Templeton-Knight: Chemistry and Industry (August) 512 (1990)). However such a procedure may mask the other properties of talc (for example its sheet structure). Also, and above all, the lack of any chemical bond between the envelope and the talc particle does not in general make it possible for the final mechanical properties to be increased significantly in comparison with those obtained using a non-encapsulated talc filler, particularly so when the envelopes of synthetic material generally provided are fragile and have little thermal stability. Patent EP-A-0.050,936 describes a process of coating with magnesium hydroxide obtained by the action of an aqueous solution of caustic soda on the talc at low temperature (below 65.degree. C. and generally at ambient temperature). The magnesium of the talc is dissolved in the solution and precipitates on the particles in the form of magnesium hydroxide. This deposit makes the substance apparently hydrophilic, but is very easily removed by e.g. acid washing, ultrasound, etc., and the initial particles of hydrophobic talc are then left. As before, a product of this kind has no stability.
Furthermore, patents GB-A-2 211 493 and JP-A-0.113.018 (Chemical Abstracts, vol. 111, no. 26, 25.12.1989, Columbus, Ohio, U.S., abstract no. 236090R, page 180) describe a process of treating talc with phosphoric compounds such as phosphoric or pyrophosphoric acids. This process produces a deposit of phosphate around the talc particles which gives them apparently hydrophilic properties. As before, this deposit is unstable and easily removed, particularly by washing in bases, ultrasound, etc.
In the case of the substances with an apparently hydrophilic nature described previously the hydrophobic nature of the initial talc is merely masked by a peripheral deposit which is not incorporated into the crystalline structure of the talc and which can easily be removed. This being the case, it is clear that the hydrophilic property conferred will be very labile, and it will not be possible to graft external molecules onto this deposit in order to obtain a complete particle with good cohesion (in which the external molecules are stably bound to the talc).
Furthermore, where the talc is used as a reinforcing filler for thermoplastics materials it has been found that the talc undergoes a thermal transformation into a mixture of enstatite and silica which forms a stable filler which significantly increases the mechanical properties of the thermoplastics obtained (see patents FR 2.527.620 and FR 2.585.691). However, such a conversion (described as "calcination") is very costly because of the high temperatures required (in practice between 1150.degree. C. and 1220.degree. C.), which need a specific technology (directly heated rotary furnaces, which become essential above 900.degree. C.). Furthermore and above all, this conversion results in a different product which is no longer talc and no longer has its properties (in particular it has a very hard crystalline structure). This product has certain specific characteristics (in particular an abrasive property) which restricts its applications as a filler in thermoplastics.